Abstract

A database of dynamic characteristics of woodframe buildings was developed through analysis of recorded earthquake response and by forced vibration and shaketable
testing. Modal identification was performed on eight sets of strong-motion records obtained from five buildings, and forced vibration tests were performed on five other buildings. The periods identified were sensitive to the amplitude of shaking, due to the reduction in lateral stiffness at stronger shaking levels. The equivalent
viscous damping ratios were usually more than 10% of critical during earthquake shaking. A regression analysis was performed on the earthquake and forced vibration
test data to obtain a simple, but reasonably accurate, period formula for woodframe
buildings at low drift levels (less than 0.1%). Data obtained from the UC San Diego
and UC Berkeley full-scale shake-table tests illustrate the shift in periods due to increasing
shaking amplitude. Forced vibration tests of the UC Berkeley 3-story building
before and after the shake-table tests showed how the periods and modeshapes
shift due to damage. A simple analytical model of masses and springs was used to
model the UC Berkeley test structure. The effects of diaphragm stiffness and mass
distribution assumptions were evaluated and found to have a significant effect on the
model torsional response. This model was used to find the equivalent wall stiffnesses
giving frequency-response curves that best-fit the experimental data. These spring
values were used to quantify the stiffness loss resulting from severe shaking of the
structure, and the observed damage corresponded to stiffness losses of over 75%. The
correlation between stiffness loss and damage to woodframe buildings has potential
structural health monitoring implications.